Heat transfer equation - formulation

Hi, I have a device which is heated from inside by constant heat (let's say by Joule's heat - UIt). Some heat causes temperature rise of the device, some is lost into surroundings through device's surface. I measured how temperature of the device varies with time and I want to calculate constants like heat capacity of the device and heat loss coefficient.

I'm confused about last expression in the equation: What does dT in the expression γ(dT2)dt mean? Is it the same as dT1 or is it rather instantaneous temperature of the device minus temperature of surroundings?